IMED1001 - Electrophysiology

Summary of concentration of ions in cells and outside cells

- in intracellular fluid of skeletal muscle: a lot more K+ then Na, high conc. of PO4 3-

- in Plasma and interstital fluid: more Na+ then K+. A lot of HCO3- and Cl-

Na+/K+ ATPase

A protein found in the plasma membrane of all cells in the body that uses the energy of an ATP (hydrolyzes ATP) to move three Na+ ions out of the cell and two K+ ions into the cell, thus establishing concentrations gradients for these ions across the cell membrane.

SUMMARY:

- 3 Na+ out per 2 K+ in

-uses 1 ATP

- not iso-electric

Factors affecting diffusion of ions

- membrane permeability

- concentration gradient

- voltage gradient

Resting Membrane potential

- Cytoplasm of typical cells is electrically negative (around -70mV) with respect to the extracellular fluid

negative because:

- there is asymmetric ion distribution established/maintained by active transport

- Membrane is around 50-75x more permeable to K+ then Na+ (K+ leakage channels), hence negatively charged

- Gibbs-Donnan Effect makes inside more negative because: macromolecules assembled inside cells which means that small building blocks diffuse in. Most proteins ionise as -ve ions so -ve ions stuck in cell, hence -ve charge

Types of Ion Channels (3 of them)

- Leak Channels: always open (like aquaporins)

- Ligand-Gated Channels: open or shut when bound by a specific ligand (chemical messenger)

- Voltage-Gated channels: open at specific membrane potential, close at a specific membrane potential

Local Potential

an electrical potential that is initiated by stimulation at a specific site, which is a graded response that spreads passively across the cell membrane, decreasing in strength with time and distance

- passive local potentials don't stimulate action potential

- amplified local potentials stimulate action potential

Excitable cells

- neurons and muscle cells are electrical excitability

- they alter their membrane potential to transmit signals

GO BACK TO LEARNING ABOUT HOW ACTION POTENTIALS WORK FROM YEAR 12 HBIO

- the slides in IMED1001 go heavily through action potentials. The following info after this flashcard will be extra info you need to know. Everything else comes back to year 12 HBIO nervous system

Channels involved in Action potential

- Ligand-gated Na+ channels: Open when bound by a specific ligand (chemical messenger). Provide initial stimulus

- Voltage-Gated Channels (Na+): open when membrane potential voltage exceeds threshold (-55mv). close a few ms later. inactivated for a few ms afterwards

- Voltage-Gated K+ channels: open due to membrane potential voltage exceeding threshold. Slower to open then Na+ channels (open later). Close a few ms later

Positive Feedback in Action potential depolarisation

- sub-threshold stimuli cause some Na+ channels to open, some Na+ enters cell in turn causing further depolarisation

- Once threshold membrane potential is exceeded, all voltage-gated Na+ channels open.

- Na+ floods into cell down its electrochemical gradient, causing rapid rise in membrane potential

Repolarisation

- at peak membrane potential, cell is highly positively charged compared to ECF

- once cell MP peaks, voltage gated Na+ channels close and voltage gated K+ channels open

- K+ floods out of cell down electrochemical gradient causing repolarisation

Hyperpolarisation

- After repolarisation, low K+ in cell and Na+ is being pumped out

- K+ channels are slower to close, so K+ leaks back out of the cell

- Causes ICF to be more negative than at rest (until all K+ channels close)

Refractory Period

- Na+ channels are inactivated upon closing: cannot open, slow K+ channels decrease MP

- Absolute refractory period: neuron can not be made to reach AP, all Na+ channels inactivated

- Relative refractory period: neuron can generate AP, but requires greater stimulus. Some Na+ channels re-activated

- After action potential, neurons need to recover: re-establish polarisation. Na+/K+-ATPASE pumps Na+ out of cell and K+ in

Propogation of Action potentials

- action potentials do not diminish over distance, they spread from one membrane microdomain to the next microdomain - called propogation

- action potential starts at the trigger zone

Schwann Cell and Node of Ranvier

- revise these from Human BIo Unit 3 Nervous system

Saltatory Conduction

- myelin (fat) insulates nerves

- myelin is secreted by glial cells (oligdendrocytes in CNS, Schwann cells in PNS)

- gaps between myelin are the only places where action potentials can occur

- they jump from Node to Node

- increased nerve diameter means more myelin and hence faster impulse conduction